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Old 10-06-2011, 11:28 PM   #1
velleguy73
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Default What is the best Engine/Tranny combination for a commuter?

I have a 3rd gen SOHC stick and I am rewarded with 32-34 mph no matter how hard I drive it. I drive with traffic on the highway and I'm not shy to get on it at a light. I love my SOHC and I am used to the low power. It's there, you just have to downshift to get it when you need it.

I've been thinking of picking up an extra car. There's plenty to choose from but I have been reading several threads of DOHC cars with either the 5th gear of the complete tranny from a SOHC model. From what I have read, those who made the swap have been rewarded with good fuel economy. I can live without cruise control. Considering the DOHC tranny is roughly 700rpm higher at 70mph it sounds like a smart move.

From the efficiency and fuel economy point of view, would get better numbers from a DOHC car with a tranny swap or just stick with the SOHC drivetrain? A DOHC is going to breathe better but I believe they have different fuel injectors which are likely tuned to spray more fuel.

Should I stick with what I know or build a composite drivetrain? Any thoughts or experience?

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Old 10-07-2011, 01:55 AM   #2
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Default Re: What is the best Engine/Tranny combination for a commuter?

The best combination, for mileage, is an SOHC with an SOHC manual transmission. It has nothing to do with injectors: on both engines (and virtually any modern car engine), the amount of fuel injected is precisely controlled by the array of sensors to match the amount of air passing through the engine. The reason the SOHC gets better mileage is because it's less powerful. Because of this, the throttle must remain open further, for the same effect (ie. maintaining a constant speed). This may seem counter-intuitive, but the further open the throttle, the better the mileage (when putting out the same amount of power) because one of the big losses in a gasoline engine is pumping losses because of the vacuum created by the mostly closed throttlebody. This is one of several reasons diesel engines get better mileage--they have no throttle body--power is controlled only by the amount of fuel injected.

If you want to further improve your mileage (and don't mind sacrificing what little power you have) search for "hot air intake". Essentially, you re-route the intake to suck air directly off the exhaust manifold. The warmer, less dense air means that the throttle must be further open to suck in the same mass of air. The IAT (intake air temperature) sensor will notify the PCM that the air is warm, and it will automatically adjust the amount of fuel injected to prevent running rich (and, of course, the O2 sensor fine tunes everything to maintain a perfect ratio).

The most important thing you can do, to get good mileage, is to keep up on repairs/maintenance, and drive gently. If you alter your driving style, you can probably get far better mileage than you do, now. It's not at all uncommon for SOHCs to see highway mpg numbers in the low to mid 40s. I have an automatic DOHC (the worst combination for mileage) and on numerous occasions, I've seen 40-42mpg on extended highway trips.

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Old 10-07-2011, 11:06 AM   #3
campus189
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Default Re: What is the best Engine/Tranny combination for a commuter?

Agreed.

I Have 2 identical saturns.
One manual, one standard.
Standard get 42MPG
Auto 30MPG
Both have same engine, both same year make and model.

...
1997 Saturn SL2 144,000 Miles
1997 Saturn SL1 254,000 Miles
1997 Saturn SL1 157,000 Miles
1996 Saturn SL1 97,000 Miles

Also known as FiremanCV on YouTube

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Old 10-07-2011, 01:21 PM   #4
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Default Re: What is the best Engine/Tranny combination for a commuter?

Wow, I thought a SOHC/5spd would do better.

What is your driving mix?

My '02 DOHC/Auto delivered consistent 35mpg with my commute -- 120 miles round trip, mostly highway.

The '00 DOHC/5spd delivers 37 & 38, but that depends on The Traffic Gods.

I shift like your grandma and keep it at 70 or below as much as possible, but 75 does happen now and then to keep death threats to a minimum.

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Old 10-07-2011, 09:16 PM   #5
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Default Re: What is the best Engine/Tranny combination for a commuter?

Ya, I totally agree. I had a dohc auto for a while and the best mpg I got was 28. Doing the speed limit through Ohio I booked 42 in 1 long trip. Detroit is either stop n go traffic or highway. That 70mpg limit is kind of a suggested starting point. Folks tend to just go with traffic.

Interesting point about WOT. I can't say I have really thought about it that way before. Kind of makes sense considering the best fuel economy sweet spot is not idle. At idle, it is sucking air through a straw. Considering a DOHC has a more efficient head, it would be a nice combination with a transmission with a proper highway gear.

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Old 10-07-2011, 10:42 PM   #6
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Default Re: What is the best Engine/Tranny combination for a commuter?

SOHC + 5 spd, 98+ if I had to choose/bet

The hot air intake will give an extra 15% MPG at the cost of performance. I haven't tested it yet with a DOHC but probably someone has by now.

I have a 99 DOHC that gets 41 around town

I have a 96 DOHC that gets 37 around town

my 97 SOHC used to get 45 typically with the hot air intake, but the big difference was that I didn't need to try hard to get 45. I would see 70's at times on the scangauge on it.

I'm looking for an SOHC trans for my DOHC, and will build an improved hot air intake for it as well. I don't think it will ever get 55 mpg at 55 mph like the 97 did, but it will do well enough that I will try not to lose sleep over it.

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Old 10-08-2011, 12:25 AM   #7
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Default Re: What is the best Engine/Tranny combination for a commuter?

Personally I'd go with the DOHC engine and do a 5th gear swap from one of the later MP2 transmissions. I've seen nothing to convince me to believe that in daily driving the single cam engine is any more fuel efficient, it's just that the transmission ratios are more favorable for good MPG.

I drive a single cammer now, and even with the lesser power these cars seem to be real efficient at lower revs, both cruising and to some extent in stop and go as well. Being that the DOHC engine makes a little more torque even lower in the power band could easily save a downshift now and then, and combined with the lower ratios of the tranny I think the MP3 1-4 ratios with the taller 5th gear would work out just as good as the single cammer engine and tranny.

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Old 10-09-2011, 01:15 PM   #8
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Default Re: What is the best Engine/Tranny combination for a commuter?

Anybody have a DOHC with a 5th gear or full tranny swap done? Would you recommend it?

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Old 10-09-2011, 02:58 PM   #9
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Default Re: What is the best Engine/Tranny combination for a commuter?

The 5th swap on the DOHC trans makes the most sense. I happen to like cruise though so I will just leave it as is.

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